OB Spectral Group Classifications as Distance Indicators.
Arthur R. Upgren · The Astronomical Journal · 1962
Beer at the last meeting of the American Astronomical Society at Cambridge (Astron. J. 67, 268, 1962), it was shown that the R values derived from photoelectric narrow-band measurements of H~ in early-type stars provide a rapid method of luminosity determination. The 434 stars examined are mostly brighter than the eighth magnitude. The aim of this paper is to investigate the possibility of a close correlation between these R values and the OB classifications of Slettebak and Stock (7. Astrophys. 42, 67,1957) which extend down to a limiting photographic magnitude of about 13. They divide early-type stars into three main groups known as OB+, OB, and OB-, using objective prism plates of the Hamburg Schmidt telescope. If such a correlation exists for those brighter stars which are at present common to both systems, then it should also hold for stars between magnitudes 8 and 13. In order to test this correlation, only stars can be used in which H~ is free of emission. Such emission stars have been eliminated from consideration. Altogether 30 stars from Beer's list have available OB spectral types. These yield the following results: for OB+, n=5 and R=1.037+0.010; for OB, n=15 and R=1.063+0.008; and for f)B-, n=10 and R=1.126+0.006, where n and R are the number of stars and the mean R values, respectively, for each spectral group. The calibration curve of R, i.e., the corresponding absolute magnitudes, derived from 130 standard stars as reproduced in Beer's more detailed paper (Monthly Notices Roy. Astron. Soc., to be published) gives: - 5.9+0.6, -4.6+0.3 and - 2.5+0.2, respectively, for the three R values quoted above. In view of the intrinsic accuracy of this calibration of R vs A', the assignment of these luminosities, and hence distances, to the stars in the three OB groups seems justified. The statistical application of this niethod to the Hamburg-Cleveland Luminous Star Surveys should provide further insight into galactic spiral-arm structure.